In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB”, “B node”, or (in LTE) eNodeB. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions (particularly earlier versions) of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are performed by the radio base stations nodes. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
The evolved UTRAN (E-UTRAN) comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing evolved UTRA user-plane and control-plane protocol terminations toward the wireless terminal. The eNB hosts the following functions (among other functions not listed): (1) functions for radio resource management (e.g., radio bearer control, radio admission control), connection mobility control, dynamic resource allocation (scheduling); (2) mobility management entity (MME) including, e.g., distribution of paging message to the eNBs; and (3) User Plane Entity (UPE), including IP Header Compression and encryption of user data streams; termination of U-plane packets for paging reasons, and switching of U-plane for support of UE mobility. The eNB hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. The eNodeB also offers Radio Resource Control (RRC) functionality corresponding to the control plane. The eNodeB performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.
Only the Packet Switched (PS) domain will be supported by Long Term Evolution (LTE), i.e. all services are to be supported in the PS domain. The Long Term Evolution (LTE) standard will be based on multi-carrier based radio access schemes such as Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and SC-FDMA in the uplink. Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
In the time domain, one subframe of 1 ms duration is divided into 12 or 14 OFDM (or SC-FDMA) symbols, depending on the configuration. One OFDM (or SC-FDMA) symbol comprises a number of sub-carriers in the frequency domain, depending on the channel bandwidth and configuration. One OFDM (or SC-FDMA) symbol on one sub-carrier is referred to as an resource element (RE). See, e.g., 3GPP Technical Specification 36.211, V8.3.0 (2008 May), Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and Modulation (Release 8), which is incorporated herein by reference in its entirety.
In Long Term Evolution (LTE) no dedicated data channels are used, instead shared channel resources are used in both downlink and uplink. These shared resources, the downlink shared channel (DL-SCH) and the uplink shared channel (UL-SCH), are each controlled by one or more schedulers that assign(s) different parts of the downlink and uplink shared channels to different UEs for reception and transmission respectively.
The assignments for the downlink shared channel (DL-SCH) and the uplink shared channel (UL-SCH) are transmitted in a control region covering a few OFDM symbols in the beginning of each downlink subframe. The downlink shared channel (DL-SCH) is transmitted in a data region covering the rest of the OFDM symbols in each downlink subframe. The size of the control region is either, one, two, three or four OFDM symbols and is set per subframe.
Each assignment for downlink shared channel (DL-SCH) or uplink shared channel (UL-SCH) is transmitted on a physical channel named the Physical Downlink Control Channel (PDCCH) in the control region. There are typically multiple PDCCHs in each subframe, and each wireless terminal (UE) will be required to monitor the PDCCHs to be able to monitor a subset of the Physical Downlink Control Channels (PDCCHs).
A Physical Downlink Control Channel (PDCCH) is mapped to (e.g., comprises) a number of control channel element (CCEs). Each control channel element (CCE) consists of thirty six resource elements (REs) A Physical Downlink Control Channel (PDCCH) comprises an aggregation of 1, 2, 4 or 8 control channel element (CCEs), See, e.g., 3GPP Technical Specification 36.213, V8.3.0 (2008 May), Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8), which is incorporated herein by reference in its entirety. These four different alternatives are herein referred to as aggregation level 1, 2, 4, and 8, respectively. Each control channel element (CCE) may only be utilized on one aggregation level at the time. The variable size achieved by the different aggregation levels is used to adapt the coding rate to the required block error rate (BLER) level for each Physical Downlink Control Channel (PDCCH). The total number of available control channel element (CCEs) in a subframe will vary depending on several parameters like number of OFDM symbols used for PDCCH, number of antennas, system bandwidth, Physical HARQ Indicator Channel (PHICH) size, etc.
Control channel elements (CCEs) and their constituent resource elements (REs) are spread out, both in time over the OFDM symbols used for PDCCH, and in frequency over the configured bandwidth. This is achieved through a number of operations including interleaving, and cell-specific cyclic shifts etc. These operations also serve the purpose of randomizing the mapping between different cells. All these operations are entirely known to the wireless terminals (UEs).
The ODFM symbols used for the Physical Downlink Control Channel (PDCCH) are the ones in the control region of each subframe. If more than one OFDM symbol is used for the PDCCH (e.g., two, three or four), the PDCCH resource elements (REs) of different OFDM symbols will differ in some aspects
In the very first OFDM symbol for a PDCCH, the amount of resource elements (REs) left for control channel elements (CCEs) is reduced due to the reference signals (RSs) and symbols for PCFICH which are also in the first OFDM symbol. Furthermore, the PHICH is also allocated resources only in the very first OFDM symbol for PDCCH, in the case of normal PHICH duration being configured. Also, multiple antenna transmission may require that additional resource elements (REs) are made unavailable for PDCCH control channel elements (CCEs). Also these resource elements (REs) are present in the first or potentially first two OFDM symbols, depending on the number of configured transmit antennas.
Furthermore, the resource elements (REs) corresponding to a certain control channel element (CCE) are spread over the OFDM symbols used for the control region in a random fashion. This is achieved by using a fixed interleaving pattern followed by a cell specific cyclic shift.
Resource elements (REs) available for control channel elements (CCEs) are typically not fully utilized for transmission. This is a result of limited UE search spaces and the rough granularity of the configurable number of control channel elements (CCEs), where for a certain configuration, there are at most three different alternatives, corresponding to three different numbers of OFDM symbols used for PDCCH.
Resource elements (REs) for reference signals (RSs) and PCFICH, on the other hand, are utilized for transmission by definition. Also PHICH has another utilization pattern compared to the control channel elements (CCEs) for PDCCH transmission. Moreover, there are the resource elements (REs) which per definition are unused due to reference signal transmission on another antenna port. The impact of these unused resource elements (REs) is visible especially for the second OFDM symbol in case of four antenna ports being configured for transmission.
When it comes to selecting control channel elements (CCEs) for a certain purpose, e.g. for a down link assignment for a certain UE, a number of consecutive control channel elements (CCEs) fulfilling certain restrictions must be selected for this purpose. There are often several possible choices for the control channel element (CCE) selection. Typically the conventional control channel element (CCE) selection approach has been random, routine, or rigid (e.g., lowest CCE index, etc.). In at least some situations the conventional control channel element (CCE) selection techniques are improvident.
For example, using conventional CCE selection techniques the total load, in terms of utilized resource elements (REs), in the different OFDM symbols used for control could vary substantially. This is a problem for different reasons. One reason is that the power available for usage in each OFDM symbol is limited. Furthermore, power not utilized in one OFDM symbol can not be utilized in a later OFDM symbol, but is lost in a sense. Another potential problem is that the load variations of different OFDM symbols causes an uneven interference balance between different OFDM symbols. Assuming the same power spectral density (PSD) for different type of resource elements (REs), the inter cell interference would typically be more severe in the first OFDM symbol for PDCCH transmission compared to potential later ones. By introducing power control functionality, e.g. reference signal (RS) or PCFICH power boosting, the effects of uneven power distribution among different PDCCH OFDM symbols could increase even further. This may reduce an already limited PDCCH capacity, resulting in an even lower system capacity.